The present invention relates to a system for reproducing a magneto-optical recordable disc such as Minidisc of SONY, and more particularly to a scanning system.
Today, the Minidisc (MD), which is capable of random access, as well as the CD, has become popular. One type of the MD is only for reproduction and is manufactured in the same manner as the CD. Another type of MD is a recordable MD made of magneto-optical disc, capable of over-writing by a magneto-optical modulation recording method. Namely, in order to write data on the disc, a laser beam is radiated from under the disc to elevate the temperature thereof, and a magnetic field is applied from above to reverse the magnetic field of the recording surface. Hence the recordable MD cartridge has a shutter having an access window on either side thereof.
The main feature of the MD is its full function of random accessibility. For example, when polycarbonate is injection molded to form a substrate of the MD, a pregroove is also formed along the entire track of the disc so that a track-following servo system and spindle servo system are utilized at recording and at reproduction.
Addresses are recorded along the entire track of the MD so that a stable and quick random access is possible even in the case of unrecorded disc. Hence, editing of the disc is much simplified.
Referring to FIG. 8, a disc player for reproducing a CD 1a has a pickup 2 for reading out data recorded thereon. The read out data are amplified by an R/F amplifier 3 and fed to a digital signal processor 5 through a servo processor 4. The data are further fed to a D/A converter 7 so as to be converted into an analog signal. The analog signal is amplified by an amplifier 8 and outputted through a loudspeaker. The R/F amplifier 5, servo processor 4, and digital signal processor 5 are controlled by a CPU 6.
In such a player, a scanning for quickly reproducing the recorded data is performed by depressing a fast forward button or a reverse button while playing a CD. Namely, as shown in FIG. 9, when the scanning is instructed while playing at a step 201, the player reproduces the data for a very short time such as several tens of milliseconds at a step 202. At a step 203, the pickup 2 of the player jumps to the adjacent track, and then again reproduces the data for several tens of milliseconds. The steps 202 and 203 are repeated so that the data are intermittently reproduced as shown in FIG. 10.
Referring to FIG. 11, a conventional player for an MD 1 has a memory controller 9, a dynamic random access memory (DRAM) 10, and an adaptive transform acoustic coding (ATRAC) 11 in addition to the CD player circuitry shown in FIG. 8. That is, the reproduced data processed at the digital signal processor 5 is written in the DRAM 10 by the memory controller 9. Audio data recorded on the MD 1 is not always regularly arranged. The data of the disc are read out in accordance with the logical arrangement information and written in the DRAM 10. The stored data are read out by the memory controller 9 and fed to the ATRAC 11 to be decoded. The decoded data are thereafter converted into an analog signal by the D/A converter 7 and fed to a loudspeaker through the amplifier 8.
The use of the DRAM is advantageous in preventing skipping of sounds which occur when the pickup 2 jumps by a shock. More particularly, the pickup reads digital data recorded on the MD 1 at the speed of 1.4 Mbit/sec and the read-out data are written in the DRAM. On the other hand, the ATRAC 11 decodes the data at the speed of 0.3 Mbit/sec so that the analog signal is produced at the D/A converter 7 and reproduced without interruption if the jump occurs. Supposing that the capacity of the DRAM 10 is one Mbit, while the MD is played, the DRAM becomes full in 0.9 second. When a jump of the pickup occurs, it becomes impossible for the pickup to read the data. However, since the data stored in the DRAM 10 is fed to the ATRAC 11 for another three seconds, if the pickup quickly returns to the position where the data are not read in three seconds and resumes the reading, there is no interruption in the audio signal fed to the loudspeaker. Hence the DRAM is sometimes called a shock proof memory or a data buffer. The position where the data are not read can be easily determined by checking the addresses which are recorded along the entire track of the MD.
As shown in FIGS. 12a to 12d, the DRAM 10 is a ring buffer. Referring to FIG. 12a, when the MD 1 is played, the data is read and written in the DRAM 10 starting from an area a, that is the hatched portion in the figure, and further the writing continues in a clockwise direction. After a predetermined time, the data are read out from the area a and fed to the loudspeaker to generate sound while data are written at an area b of the DRAM. Namely, as shown in FIG. 14a, the data in the DRAM are read out at a point A of the MD 1 to emit a sound, although the pickup 2 has already proceeded to a point B, reading and writing the data thereat.
When the entire recording area of the ring is written as shown in FIG. 12c, the data are written again in the area a whereas the data are read from an area c for generating sound. When the fast forward button is depressed for fast forward scanning after the DRAM 10 becomes full, all of the stored data which should be reproduced in a normal reproduction are erased (see FIG. 12d). Hence, no further data are read out from the DRAM 10. The pickup 2 at the point B of the MD 1 in FIG. 14a then jumps back to the point A where the data are being read out and reproduced at the request of the fast scanning. Thereafter, a program shown in FIG. 13 is started. More particularly, at the point A of FIG. 14b, data on the disc are read out for several tens of milliseconds and written in the DRAM, and reproduced. The pickup 2 then jumps to the adjacent track and the data on the disc are read and written and the data stored in the DRAM 10 are read for generating sound. The skip writing and reproducing are repeated so that the scanning is performed.
In such a conventional player, the pickup must return to the position where the data are actually being output as sound, and the data must be written before the sound is emitted. Thus the operational response of the player is poor.